1. Field of the Invention
The present invention relates to a step-up/step-down switching regulator supplying power to a load by converting input direct voltage from, for example, a battery, to a predetermined constant voltage for use in various electronic devices.
2. Description of the Related Art
FIG. 3 is a circuit diagram showing an example of a conventional step-up/step-down switching regulator (see, for example, Japanese Laid-Open Patent Application No. 11-299229). FIG. 4 is a timing chart illustrating examples of waveforms of respective parts in the circuit of FIG. 3.
In FIG. 3, the voltage difference between a predetermined reference voltage Vref and a divided voltage Vfb obtained by dividing an output voltage Vo with resistors R111, R112 is amplified by an error amplifier circuit including an operational amplifier circuit 111 and resistors R113, R114. The voltage Vb output from the operational amplifier circuit 111 is input to a step-up PWM comparator 117 and compared with a triangular wave voltage Vt, to thereby generate a pulse-width modulated signal. Then, a step-up drive circuit 118 controls the on/off switching of a step-up switching transistor Tr 112 according to the pulse-width modulated signal.
Further, a level shift circuit including an operational amplifier circuit 113, resistors R115-R118, and a shift voltage generating circuit 114 subtracts a shift voltage Vs from the voltage Vb output from the operational amplifier circuit 111, to thereby obtain a subtracted voltage Va. Then, the subtracted voltage Va is input to a step-down PWM comparator 115 and compared with the triangular wave voltage Vt, to thereby generate a pulse-width modulated signal. Then, a step-down drive circuit 116 controls the on/off switching of a step-down switching transistor Tr 111 according to the pulse-width modulated signal.
With reference to FIG. 4, in a case where the voltage amplitude of the triangular wave voltage Vt is “V1”, the voltage Vb and the voltage Va will not intersect the triangular wave voltage Vt at the same time when the shift voltage is equal to or greater than V1. The voltage Va becomes equal to or less than a lower limit voltage of the triangular wave voltage Vt when the voltage Vb is within the amplitude range of the triangular wave voltage Vt. The voltage Vb becomes equal to or greater than an upper limit voltage of the triangular wave voltage Vt when the voltage Va is within the amplitude range of the triangular wave voltage Vt.
Accordingly, in a case of a step-up operation, the step-down switching transistor Trill is switched on to attain a conduction state. In a case of switching from the step-up operation to a step-down operation, the on-duty cycle of the step-down switching transistor Tr111 is gradually reduced after the on-duty cycle of the step-up switching transistor Tr112 becomes 0%.
Likewise, during the step-down operation, the step-up switching transistor Tr112 is switched off to attain a shut-off state. In a case of returning from the step-down operation to a step-up operation, the step-up switching transistor Tr112 is switched on after the on-duty cycle of the step-down switching transistor Tr111 becomes 100%.
In FIG. 3, since the level shift circuit subtracts a shift voltage from the voltage Vb output from the operational amplifier circuit 111 of the error amplifier circuit, the upper limit voltage of the range of voltage output from the operational amplifier circuit 111 is required to be at least equal to or greater than a voltage obtained by adding the amplitude voltage V1 of the triangular wave voltage Vt to the upper limit voltage of the triangular wave voltage Vt.
FIG. 5 is a circuit diagram showing another example of a conventional step-up/step-down switching regulator (see, for example, Japanese Laid-Open Patent Application No. 11-299229).
In FIG. 5, the level shift circuit of FIG. 3 is configured to add a shift voltage Vs to a voltage Va output from the operational amplifier circuit 111 used in the error amplifier circuit. Therefore, in the example shown in FIG. 5, the output voltage Va of the operational amplifier circuit 111 is directly input to the step-down PWM comparator 115, and the output voltage Vb of the operational amplifier circuit 113 used in the level shift circuit is input to the step-up PWM comparator 117.
Although operation of the circuit shown in FIG. 5 is similar to that of the circuit shown in FIG. 3, since the level shift circuit performs the above-described addition process, the lower limit of the voltage output from the operational amplifier circuit 111 is required to be at least the amplitude voltage V1 of the triangular wave voltage Vt lower than the lower limit voltage of the triangular wave voltage Vt.
In FIG. 3, the step-up operation is performed by inputting the output voltage Vb of the error amplifier circuit to the step-up PWM comparator 117. Generally, in a case of performing the step-up operation, the frequency characteristics, which are determined according to an inductor and a smoothing capacity, become unstable (e.g., greater oscillation compared to the oscillation during the step-down operation) since the phase of the step-up operation, unlike that of the step-down operation, is delayed 180 degrees. Therefore, phase compensation for stabilizing the step-up operation is performed, for example, by connecting a condenser to a feedback loop. However, this causes the capacity of phase compensation to become greater than that of the step-down operational amplifier circuit. Therefore, the frequency characteristics of the operational amplifier circuit 111 are restrained (particularly, high frequency characteristics degrade significantly). In a case where the output voltage Vb of the operational amplifier circuit 111 having the degraded high frequency characteristics is input to the level shift circuit, the high frequency characteristics of the output voltage Va of the level shift circuit degrade further. Thus, the high frequency characteristics during the step-down operation remain degraded. This adversely affects the response time for both the step-up operation and the step-down operation.
In the circuit illustrated in FIG. 5, since output voltage Va of the error amplifier circuit is input to the step-down PWM comparator 115, the amount of phase compensation of the operational amplifier circuit 111 used in the error amplifier circuit can be reduced, and the degrading of high frequency characteristics can be reduced. However, since voltage Vs is added to the output voltage Va of the operational amplifier circuit 111 for performing the step-up operation, operations are always started from the step-up operation when power is turned on. Since the step-down switching transistor Tr111 is switched on during the step-up operation, an output condenser C112 is charged with a large amount of current from an input voltage via the step-down switching transistor Tr111, an inductor L111, and a rectifier diode D112 when power is turned on. Thus, a large inrush current is generated when power is turned on. Therefore, the circuit of FIG. 5 needs to control incoming current by providing, for example, a soft start circuit for restraining inrush current.